Maple Syrup: canning or freezing

Sometimes people buy maple syrup in large quantities.

If not used promptly or stored properly, these bulk purchases can often become spoiled by being colonized with mould.

This article covers techniques for storing maple syrup at home.

To be clear, this page is not directed at commercial maple syrup producers / packers. They should look to associations and extension services in their regions for professional advice and guidelines that will take into account their circumstances and volumes.

Spoiler alert: the preferred long-term storage method for home preservation is freezing.

See also: Maple syrup, Maple syrup mould (links open on CooksInfo.com), Maple Walnut Syrup

Shelf storage

Unopened metal, glass or squeeze-type plastic containers of maple syrup straight from the store or producer and whose seal has not been broken will have a long shelf life at normal room temperatures in the cupboard.

Unopened beige / grey plastic jugs, however, should be stored in the fridge, even if still sealed. The Quebec Maple Syrup Producers Association says:

“Maple syrup (if it’s been boiled and packaged according to our standards) keeps at room temperature for a very long time in most containers, including glass or squeeze-type plastic bottles. However, tests have shown that some receptacles, while quite lovely, do not provide foolproof barriers to oxygen. That’s why we recommend that you put that beige plastic jug with the maple scenery on it into the fridge. It’ll prolong the life of your maple syrup.” ^[1]Quebec Maple Syrup Producers (QMSP) Association. Keeping your maple syrup. Accessed February 2021 at https://maplefromcanada.ca/products/maple-syrup/

What keeps shelf-stored maple syrup safe?

To legally quality as maple syrup in North America, a certain amount of water has to be driven out of maple sap, reducing it down to a syrup with a certain (high) concentration of sugar in it.

The North American Maple Syrup Council says,

“The correct density for maple syrup is between 66° and 68° Brix, with some local jurisdictions that have strict maple laws requiring a narrower range… If your finished product is below 66° Brix it will likely become moldy and ferment, and if it is above 68° Brix it will form sugar crystals in the bottom of the container. [One Brix is equal to about 1% sugar content]” — When is it syrup? North American Maple Syrup Council. Supplement to June 2017 Maple Syrup Digest. Page 1.

While traditional methods such as “aproning” can tell you when you are getting close to your sap turning into syrup, there are several modern measuring tools such as hydrometers that can tell you with precision.

A 1996 research project found that “the moisture content of maple syrup ranged from 26.5 to 39.4%. The pH and °Brix values for maple syrup ranged from 5.6 to 7.9, and 62.2 to 74.0°, respectively.” ^[2]Jackie G. Stuckel, Nicholas H. Low, The chemical composition of 80 pure maple syrup samples produced in North America, Food Research International, Volume 29, Issues 3–4, 1996, Pages 373-379, ISSN 0963-9969, https://doi.org/10.1016/0963-9969(96)00000-2.

The FDA gives an average pH for maple syrup of 5.15. ^[3]FDA. Approximate pH of Foods and Food Products. April 2007. Accessed August 2017 at http://www.vldhealth.org/pdf/environmentalPDF/foodPH2007.pdf

Because the pH levels of maple syrup are above 4.6, there could be a risk of C. botulinum spores “germinating” in anaerobic environments such as sealed jars. But it is the high sugar content and resultant low water activity in the syrup (when sufficient water is driven out of the sap to turn it into the definition of maple syrup) which keeps it safe.

Refrigeration

After the seal has been broken on a container of maple syrup, consider it contaminated with mould spores from the air, and therefore no longer suited to room temperature storage on a shelf. It must be refrigerated.

“Once opened, you should move your maple syrup to the fridge…. The cold temperature [of a fridge], and the low water activity, will work together to slow or prevent the mould from growing.” ^[4]Yabsley, Charmaine. Are you team fridge or team cupboard for maple syrup? Sydney, Australia: Special Broadcasting Service. 15 November 2017. Accessed March 2021 at https://www.sbs.com.au/food/article/2017/11/15/are-you-team-fridge-or-team-cupboard-maple-syrup

Refrigerating opened maple syrup will slow down mould growth, not prevent it. If you have a large quantity of maple syrup that you are slowly working your way through, there’s a chance it may spoil on you with mould over time. You may wish as a precaution to consider keeping in the fridge only what you can use in a reasonable time, and using another food preservation method to make the rest keep longer.

Freezing

At home, freezing excess maple syrup is the recommended way to preserve it long-term.

It’s not only the method that preserves quality the best, it’s also easy to do and provides complete usage flexibility.

Freezing lets you take a large amount of maple syrup, and divide it into smaller containers of a reasonable usage size for your household, and store them in a way that will preserve the quality. Freezing prevents all mould growth.

When frozen, maple syrup won’t be solid owing to its high sugar content. Instead, it will become viscous, with the thickness and texture of honey while frozen, reverting to its normal self when thawed.

This video shows the consistency of maple syrup straight out of a chest freezer:

Freezing, thawing and refreezing is safe.

“You can freeze it, going through as many freezing and thawing cycles as you want, the [Vermont Maple Sugar Makers’ Association] says, as long as you let it thaw completely and stir in any condensation that forms on the top of the syrup.” ^[5]Krystal, Becky. Before you pour all that maple syrup on your pancakes, here’s what you should know about it. Washington, D.C.: The Washington Post. 2 September 2019.

It is best frozen in glass, as plastic can allow air through, and metal might rust or give a metallic taste to the syrup. It’s probably okay to freeze it in those grey / beige jugs for a short period of time, but the plastic in those jugs is porous, letting air in, so you may experience degradation in the quality of the syrup if you freeze the syrup in them for any length of time. There is no need to sterilize the glass jars or lids first. For Mason jars with two-piece lids, for freezing purposes you can re-use canning lids (the flat parts) that were used previously in canning. (The canning rings can of course always be re-used anyway.)

Purdue University Extension Services says:

“To freeze, pour it into freezable glass jars, making sure to leave a one-inch space at the top. Always seal jars tightly. Freeze immediately.” ^[6]Maple Syrup. FoodLink. West Lafayette, Indiana: Purdue University Extension Services. 2020. Accessed February 2021 at https://extension.purdue.edu/foodlink/food.php?food=Maple%20Syrup

Ohio State Extension Service says:

“The best way to repackage maple syrup to maintain its quality is to pour the syrup into clean ½ pint, pint, or quart glass freezer jars to one inch from the top and freeze.” ^[7]Drake, Barbara H. Selecting, Storing, and Serving Ohio Maple Syrup. Ohio State University Extension. HYG-5522. 25 February 2010. Accessed March 2021 at https://ohioline.osu.edu/factsheet/HYG-5522

If you need to thaw a jar of syrup quickly, just place jar in a basin of cool or lukewarm water. Or, scoop some frozen syrup out of the jar, place in a microwave-safe jug, and zap at 10 to 15 second intervals just until liquid again. Do not bring to a boil or you may damage the quality of the syrup. Caution, do not place frozen jars in microwave or in extreme hot water.

Freezing maple syrup headspace

To clarify the headspace for syrup freezing recommendations, the idea is to allow sufficient space for expansion while freezing so that the jar doesn’t shatter, but also reduce oxygen space in the jar.

The recommendations above are a generic one inch (2.5 cm) for all sized jars. Note that the standard recommendation for freezing syrups is less headspace for jars smaller than 1 quart / litre:

Imperial

• Wide mouth pint: ½ inch
• Wide mouth quart: 1 inch
• Narrow mouth pint: ¾ inch
• Narrow mouth quart: 1 ½ inch

Metric

• Wide mouth half-litre: 1.5 cm
• Wide mouth litre: 2.5 cm
• Narrow mouth half-litre: 2 cm
• Narrow mouth litre: 4 cm

Source: National Center for Home Food Preservation, General Freezing Information: Headspace to Allow Between Packed Food and Closure

Storage life of maple syrup in freezer

There’s really no set time limit on how long you can keep it in the freezer, provided it is packaged correctly. The question would not be one of food safety, but rather quality. You may see some sources cite the usual “6 to 12 months” that is cited for freezer-life span, but as the syrup is not likely to suffer freezer burn in a hurry, other sources such as Purdue University Extension Services just say “extended period of time” because no one really knows how long:

“Unopened frozen syrup maintains quality and flavor for an extended period of time.” ^[8]Maple Syrup. FoodLink. West Lafayette, Indiana: Purdue University Extension Services. 2020. Accessed February 2021 at https://extension.purdue.edu/foodlink/food.php?food=Maple%20Syrup

Don’t forget to label and date the jars. You could even just write right on the lid with magic marker.

Your maple syrup will not develop mould while frozen. The freezing won’t necessarily kill any mould spores present, but it inactivates them by depriving them of the warmth they require to germinate and grow. ^[9]”Freezing has the potential to kill germinating spores and associated hyphae (fine fibrous strands), but may not be effective against dry spores (Florian 1994). ” — Treatment of Mould. Western Australian Museum. Accessed March 2021 at https://manual.museum.wa.gov.au/conservation-and-care-collections-2017/mould-and-insect-attack-collections/treatment-mould

Home ‘canning’ of maple syrup

Jar size choices: Your choice

Processing method: None

Headspace: 2 cm / ½ inch (?)

This process involves heating maple syrup that you bulk purchased so that it can be stored at room temperature on shelves in smaller sealed Mason jars.

Calling it “canning” is a misnomer because there is actually no real “canning” process happening, which would require heat processing of the filled jar for a tested time period. “Bottling” might be a better term.

In any event, home “canning”, “recanning”, or bottling, of maple syrup is recommended against not because of food safety reasons but rather owing to food quality reasons.

Ohio State University Extension says:

“Heating and “re-canning” the syrup can cause it to darken and change flavor.” ^[10]Drake, Barbara H. Selecting, Storing, and Serving Ohio Maple Syrup. Ohio State University Extension. HYG-5522. 25 February 2010. Accessed March 2021 at https://ohioline.osu.edu/factsheet/HYG-5522

There are other quality factors, too (see “Downsides” below).

Neither the National Center for Home Food Preservation, nor any of the University Extension Services, have any advice to offer you on this home activity as of spring 2021.

The method we’re endorsing is freezing (see above).

Nevertheless, if you wish to attempt it rather than freezing, this is what the process would involve based on advice from extension service to hobbyist and small maple syrup producers.

• sterilize the jars that the maple syrup will go into, along with the lids, as well as any equipment that the maple syrup will touch — spoons, ladles, funnels, etc.. This requires being in boiling water for 10 minutes (at sea level);
• keep the jars and lids hot;
• heat the maple syrup to a range between 82 to 85 ⁰C (180 to 185 ⁰F);
• ladle syrup into a heated jar, leaving very little headspace — say 2 cm / ½ inch;
• put the lid on each jar as it is filled and ensure it is tightly sealed;
• then quickly invert that jar;
• repeat process until all syrup is in sealed jars;
• after they have all sat for 5 minutes, allow to cool quickly.

Usage notes

The headspace is admittedly just a guess on our part. You want enough to allow some kind of vacuum to form, but not enough that you are allowing the presence of too much oxygen.

When screwing the canning rings on, ignore the usual “finger-tip tight” guideline for actual canning, and instead fully tighten them.

Be careful about burning yourself especially when inverting the jars. If the tops come loose or otherwise allow any of that scalding hot syrup to escape, you could easily end up with emergency-room grade burns.

Wear protective gloves while doing this:

“Protective gloves made of silicon, or heavy-duty rubber will provide a better grip on preheated bottles as they are filled with hot syrup.” ^[11]Leuty, Todd. Preventing mould in packed maple syrup. Toronto, Canada: Ontario Ministry of Agriculture, Food and Rural Affairs. 24 March 2014. Accessed March 2021 at http://www.omafra.gov.on.ca/english/crops/hort/news/hortmatt/2014/04hrt14a2.htm

After the 5 minutes standing, do what you can to facilitate quick cooling: spread them apart in a cool area, put in a cool window or place in the path of a breeze from a fan or air conditioner. [Yes, this goes contrary to the rules of normal home canning.] Just do not put the filled, hot jars into cold water, as obviously that kind of thermal shock could shatter the jars. For the reason why you want quick cooling, see the the QUICK COOLING section further down the page.

Ideally, you should get a vacuum seal on the jars. Note that it will be a weak vacuum seal, nowhere near as strong as you would get if you properly processed the jars in boiling water or in a steam canner, but that kind of processing will damage the quality of the maple syrup and perhaps make it unpalatable (see HEAT section further down the page).

From the resource reading below, you’ll realize that this method does not guarantee mould-free storage. This might allow some of the maple syrup to spoil and consequently you may therefore experience product loss because of this. This is another reason why freezing is recommended: mould will not develop in maple syrup while it is frozen.

Summary of downsides of attempting to ‘can’:

• can cause the colour and flavour to deepen, essentially lowering the grade of the syrup;
• overheating can cause maple sand to form in the syrup;
• lids could leak scalding-hot syrup during inversion;
• weak seal;
• it does not guarantee mould-free storage;
• there’s no real canning process happening.

Our advice remains to freeze the maple syrup rather than attempt to home ‘can’ it.

Issues in attempting to home ‘can’ / bottle maple syrup

The sections below discuss procedures and quality issues. They are extracted from publications discussing the issues for hobbyists and commercial processors. The principles would also apply to anyone attempting to home ‘can’ / home bottle maple syrup, and are possibly what an Extension Service would need to consider if ever examining the issue.

Also note that typically, it’s not considered good practice to attempt to apply commercial techniques to home food preservation methods, as the equipment, training and environment are so different. It’s for this reason that specialized resources for home practice are available. For maple syrup, however, there are no alternative resources available.

Note that all these recommendations appear to make the safety assumption that your sap is actually transformed into a syrup. As maple syrup is a low pH product, it is the low water activity (resulting from high sugar content) which makes the syrup safe for bottling. Before commercial operations do any bottling of maple syrup, they are going to measure that a sufficiently-high sugar content level has been reached — for quality, financial and safety reasons.

The hot pack process

In the maple syrup industry, it is standard for maple syrup to be “hot packed” into the retail containers it will be sold to consumers in.

To do this, the maple syrup is heated to within a range of 82 to 85 ⁰C (180 to 185 ⁰F):

“Maple syrup can be hot packed into retail containers at a temperature of 82 to 85 ⁰C (180 to 185 ⁰F)” ^[12]Ontario maple syrup production report. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 3 March 2016. Accessed March 2021 at
https://onmaplesyrup.ca/2016/03/03/ontario-maple-syrup-production-report/

Sometimes the upper limit of that temperature range is taken up to 90 or 92 ⁰C (194 to 197 ⁰F), but extreme care must be taken when approaching these temperatures as there can be quality impacts (see HEAT section further down the page).

A temperature in this range must be maintained during the filling process:

“It involves maintaining syrup temperature of 82°C or higher while filling and placing caps on the maple syrup containers. ” ^[13]Preventing Mould Growth in Maple Syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 12 February 2021. Accessed March 2021 at http://www.omafra.gov.on.ca/english/food/inspection/maple/mouldprevention.htm   “Hot packing involves maintaining syrup temperature of 82 to 90 ⁰C during filling, until containers are capped.” ^[14]Ontario Maple Syrup Production Report, April 19, 2018. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 19 April 2018. Accessed March 2021 at https://onmaplesyrup.ca/2018/04/19/ontario-maple-syrup-production-report-april-19-2018/”

Containers used must be capable of standing the heat. Even if the plastic being used is food grade, it must also be able to stand this temperature:

“Do not use containers or lids that cannot tolerate the recommended hot-pack temperature of 85 °C (185 °F). ^[15]Leuty, Todd. Preventing mould in packed maple syrup. Toronto, Canada: Ontario Ministry of Agriculture, Food and Rural Affairs. 24 March 2014. Accessed March 2021 at http://www.omafra.gov.on.ca/english/crops/hort/news/hortmatt/2014/04hrt14a2.htm

Most drink and beverage containers probably won’t be certified food grade at that temperature:

“[Plastic] containers that are commonly used for fresh milk are not designed for hot packing temperatures for maple syrup. These containers are usually cold packed, they are not hot packed and their use for maple syrup is discouraged by the provincial industry for retail sales.” ^[16]Maple Syrup Production report on March 13, 2015. 23 March 2015. Accessed March 2021 at https://onmaplesyrup.ca/2015/03/13/maple-syrup-production-report-on-march-13-2015/

Containers are filled to a minimum of 90%:

“Containers must be filled to a minimum of 90% of the container volume.” ^[17]Leuty, Todd. Preventing mould in packed maple syrup. Toronto, Canada: Ontario Ministry of Agriculture, Food and Rural Affairs. 24 March 2014. Accessed March 2021 at http://www.omafra.gov.on.ca/english/crops/hort/news/hortmatt/2014/04hrt14a2.htm

Once filled, the containers need to be immediately capped to retain heat, and to stop any more mould spores from the air landing into the container.

Inversion

After capping, a technique which is known as “inversion” is then practised. It involves tilting the container so that the hot syrup will touch all surfaces inside the container with the goal of destroying mould spores that may have drifted into the containers during the filling process.

You will note time estimates ranging from 3 to 10 minutes suggested for this:

“Once filled, tighten the clean cap securely to seal the container. Before it has a chance to cool, carefully turn the hot bottle or jug onto its side, or turn it upside down and place it back into the cardboard shipping box.  This procedure is necessary to force hot syrup into the neck and inner cap surface to help kill spores of spoilage organisms that may be present. Leave the hot containers inverted for approximately 5 minutes to ensure adequate heat exposure…” ^[18]Ontario maple syrup production report. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 3 March 2016. Accessed March 2021 at
https://onmaplesyrup.ca/2016/03/03/ontario-maple-syrup-production-report/

Inversion after a hot pack is the primary tool that the industry uses to combat mould:

“Hot packing is the most widely used mould preventative strategy to combat mould growth… Once the containers are capped, they should be inverted in order to reduce spoilage organisms that may be present in the neck of the container and cap liner.” ^[19]Ontario Maple Syrup Production Report, April 19, 2018. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 19 April 2018. Accessed March 2021 at https://onmaplesyrup.ca/2018/04/19/ontario-maple-syrup-production-report-april-19-2018/”

It’s a technique of temperature plus time:

“Hot packing is the most common method for destroying mould spores and limiting oxygen availability for mould growth… . Once filled and securely capped, each container is then inverted for 5 to 10 minutes to ensure the hot maple syrup contacts and kills all spoilage organisms that may be present in the top neck and inner lid surface of the container.” ^[20]Preventing Mould Growth in Maple Syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 12 February 2021. Accessed March 2021 at http://www.omafra.gov.on.ca/english/food/inspection/maple/mouldprevention.htm

Here the time estimate is 3 to 5 minutes:

“Try to maintain the sealed container at the hot pack temperature for a period of 3 to 5 minutes before allowing it to cool.” ^[21]Maple Syrup Production report on March 13, 2015. 23 March 2015. Accessed March 2021 at https://onmaplesyrup.ca/2015/03/13/maple-syrup-production-report-on-march-13-2015/

[Ed: research has in fact shown that not all spoilage organisms will always be killed by this hot pack and inversion process. See “Some mould may survive” section further below.]

Alert readers will note that the “inversion” technique is strongly recommend against for all other aspects of “home canning”. Reading through these sections should provide reinforce their understanding of why it is a very weak technique, and why it barely works for maple syrup even. Presumably it’s accepted for maple syrup as no better process has been suggested yet, and heat processing the filled jars would actually destroy the product.

See also: Inversion Canning

Vacuum quality

Note that the vacuum will not be a strong one. Strong vacuums are only formed through heat processing of filled, sealed containers.

“As hot packed maple syrup begins to cool, it contracts, which creates a vacuum in the head space of properly sealed containers… .” ^[22]Ontario Maple Syrup Production Report, April 19, 2018. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 19 April 2018. Accessed March 2021 at https://onmaplesyrup.ca/2018/04/19/ontario-maple-syrup-production-report-april-19-2018/”

Also note that one other claim cannot be true: “As hot packed syrup cools, it contracts and creates a vacuum, resulting in an anaerobic (oxygen-free) environment that is unsuitable for mould growth.” ^[23]Preventing Mould Growth in Maple Syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 12 February 2021. Accessed March 2021 at http://www.omafra.gov.on.ca/english/food/inspection/maple/mouldprevention.htm

There has been no canning process to drive air out of the headspace, therefore any oxygen present in the headspace when the cap / lid was placed on the jar will remain.

Cold packing

The opposite of hot packing is cold packing.

“Cold” pack has caused more spoilage of syrup than almost any other factor. Always pack syrup hot! When packed at the right temperature, the hot syrup will sterilize its container, preventing spoilage. Then, when properly sealed, a vacuum will be created, preventing contamination.” ^[24]Hopkins, Kathryn. Maple Syrup Quality Control Manual. University of Maine Cooperative Extension Service. 2010. Bulletin #7038. Page 10.

Cold packing leads to a great deal of product spoilage and is rarely practised now:

“Packing cold syrup into containers is not recommended and results in mouldy syrup.” ^[25]Maple Syrup Production report on March 13, 2015. 23 March 2015. Accessed March 2021 at https://onmaplesyrup.ca/2015/03/13/maple-syrup-production-report-on-march-13-2015/

Any cold-packed maple syrup should be immediately refrigerated or frozen.

Headspace

Mould spores need oxygen to grow. In bottling maple syrup, a goal is to minimize how much oxygen in the sealed container is available to any mould spores that survived the hot pack step.

“Generously fill the bottles with syrup to leave a small empty head space.  A small head space will minimize the amount of oxygen that will be inside the sealed container.” ^[26]Ontario maple syrup production report. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 3 March 2016. Accessed March 2021 at
https://onmaplesyrup.ca/2016/03/03/ontario-maple-syrup-production-report/

If you are using clear glass containers, you will also be able to check the headspace for condensation:

“When the bottles are cool, check the air space for accumulation of condensed water. Inverting the container again can force any condensed water back into the syrup and out of the airspace.” ^[27]Ontario maple syrup production report. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 3 March 2016. Accessed March 2021 at https://onmaplesyrup.ca/2016/03/03/ontario-maple-syrup-production-report/

Some mould may survive

The Ontario Ministry of Agriculture, Food and Rural Affairs says hot packing may eliminate, or it may just reduce, the quantity of mould spores:

“Hot packing will reduce or eliminate the incidence of spoilage mould in syrup.” ^[28]Maple Syrup Production report on March 13, 2015. 23 March 2015. Accessed March 2021 at https://onmaplesyrup.ca/2015/03/13/maple-syrup-production-report-on-march-13-2015/

The destruction of all mould spores is not guaranteed:

“The resulting container environment is oxygen free (anaerobic) which makes it unsuitable for mould growth. Hot packing does not guarantee destruction of all mould spores. Some heat-resistant mould spore populations are capable of surviving temperatures higher than 82 °C (180 F).” Ontario Maple Syrup Production Report, April 19, 2018. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 19 April 2018. Accessed March 2021 at https://onmaplesyrup.ca/2018/04/19/ontario-maple-syrup-production-report-april-19-2018/””

Note that in the paragraph above, “The resulting container environment is oxygen free (anaerobic)” is a faulty assumption. There has been no heat processing of the jar to drive air out of it. Whatever oxygen was in the headspace when the container was capped will remain in the container.

“While 82°C (180 F) is generally accepted as the threshold temperature to destroy mould spores, some species of mould spores are resistant to high temperatures.” ^[29]Preventing Mould Growth in Maple Syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 12 February 2021. Accessed March 2021 at http://www.omafra.gov.on.ca/english/food/inspection/maple/mouldprevention.htm

Previous industry assumptions have proven wrong:

“Previously, the maple industry assumed that microbial growth could not occur in syrup processed to a minimum of 66° Brix (66% sugar) because of the high sugar content and any contamination that occurred was likely due to low Brix. The industry also assumed that heating syrup to 85°C (185° F) before bottling would kill off any microbial contamination, and if the syrup was contaminated, boiling the syrup for 15 minutes would kill any contamination and threat to human health… [Despite this], occasionally maple syrup producers have found their bottles or containers of maple syrup have floating masses or obvious surface mold. Over the last three years, we have found 32 out of 34 samples of maple syrup turned into the University of Maine Cooperative Extension have been contaminated with fungi.” ^[30] Maine Agricultural Center Integrated Research/Extension Grants 2012–2013. page 1. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/resource/maine-agricultural-center-integrated-researchextension-grants-2012-2013/

Killing mould spores is a combination of both temperature and time:

“Fungal species were tested for their tolerance to heating at temperatures from 60°C to 82°C (140° F to 180° F) and most were found to survive heating at 75°C (167° F) for 3 minutes. Penicillium isolate (SVT11A) was heated in maple syrup held at 70°C (158° F) for different time intervals to determine if longer times at lower temperatures would kill spores. All spores were killed after 7 minutes at 70°C (158° F). Future studies will investigate whether holding syrup for longer periods of time at lower temperatures will kill Penicillium and Aspergillus spores.” ^[31]Maine Agricultural Center Integrated Research/Extension Grants 2012–2013. Page 3. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/resource/maine-agricultural-center-integrated-researchextension-grants-2012-2013/ 

To be clear, mould spoils the product and it should be discarded if that happens to it.

See also: Maple syrup mould

Don’t raise syrup temperature too hot

Raising the temperature range a bit may kill off more of the common mould spores:

“Research conducted by Whalen and Morselli found that syrup packed in retail containers at 93°C (199° F) did not develop fungal mats. ^[32] Maine Agricultural Center Integrated Research/Extension Grants 2012–2013. Page 3. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/resource/maine-agricultural-center-integrated-researchextension-grants-2012-2013/   “Producers who increase their packing temperature to as high as 92°C (198° F) often observe a decrease in the incidence of mould growth.” ^[33]Preventing Mould Growth in Maple Syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 12 February 2021. Accessed March 2021 at http://www.omafra.gov.on.ca/english/food/inspection/maple/mouldprevention.htm

The problem is that even these modest temperature increases can impact the quality of the syrup:

“However, at this temperature, niter or sugar sand will precipitate out, thus rendering syrup packed in glass jars visually unappealing.” ^[34]Maine Agricultural Center Integrated Research/Extension Grants 2012–2013. Page 3. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/resource/maine-agricultural-center-integrated-researchextension-grants-2012-2013/

Darkness and cloudiness can also result:

“However, as packing temperature increases, so does the likelihood of suspended solids precipitating out, creating cloudy syrup. Retaining high heat can cause syrup to darken, and producers who choose to maintain high syrup temperatures for even a short period of time should follow with rapid syrup cooling to minimize darkening of the product.” ^[35]Preventing Mould Growth in Maple Syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 12 February 2021. Accessed March 2021 at http://www.omafra.gov.on.ca/english/food/inspection/maple/mouldprevention.htm

Cloudy syrup has to be filtered:

“Temperatures hotter than 90⁰ C (195⁰ F) can cause dissolved minerals to precipitate out as solids, leading to cloudy syrup and a need to re-filter.” ^[36]Ontario maple syrup production report. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 3 March 2016. Accessed March 2021 at
https://onmaplesyrup.ca/2016/03/03/ontario-maple-syrup-production-report/

A special filtering cloth is required to deal with the sugar sand:

“When syrup is heated above about 180⁰ F (82⁰ C) degrees, minerals called “sugar sand” precipitate out. These will accumulate in the bottom of the container, but are fully harmless. Filtering the syrup is possible, but requires special filtering cloth.” ^[37]Smallidge, Peter. Cornell University Cooperative Extension. Getting Started with Small-Scale Maple Syrup Production. February 2018.

Glass jars need heating

An issue encountered with hot packing is that glass containers can absorb heat out of the syrup, lowering the temperature of the syrup just enough so that it is below that which is needed to start killing off mould spores:

“New designer containers are often made with glass that is thicker than standard containers. If not preheated, these containers can cool the temperature of hot syrup within seconds, down to temperatures that are no longer suitable to kill spores that may be present inside bottles and cap liners.” ^[38]Preventing Mould Growth in Maple Syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 12 February 2021. Accessed March 2021 at http://www.omafra.gov.on.ca/english/food/inspection/maple/mouldprevention.htm

The nature of the container used has an impact on how well the bottled syrup will hold its heat, and thus how effective it will be at inactivating mould spores:

“Our work has shown that container size has a role in determining potential contamination. The quart and pint plastic containers held syrup at higher temperatures longer than any other containers examined. Containers of one pint size and smaller did not have enough mass of hot syrup to maintain a temperature of 82° C (180° F) for at least three minutes after bottling with 85° C (185° F) syrup. The smaller 40 ml nip, 50 and 100 ml glass maple leaf and 100 ml marasca bottles had the fastest cooling and took less than a minute for the syrup to cool to below 70° C (158° F). It is likely that the maple shaped glass containers cooled syrup more quickly because of their larger surface area to volume ratio.” ^[39]Maine Agricultural Center Integrated Research/Extension Grants 2012–2013. Page 3. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/resource/maine-agricultural-center-integrated-researchextension-grants-2012-2013/ . 

Some packers have tried placing the filled, sealed jars in a moderately-hot water bath for their 5 minutes sitting time (note they say “cap above water” so that suggests that inversion wasn’t practised):

“Without preheating, mould contamination in glass containers has been a problem previously, due to failure of the hot syrup to remain adequately hot to kill spoilage spores… A few producers have tried placing the freshly sealed glass containers into a hot water bath for 5 minutes (with cap above the water level), maintained at around 82 ⁰C (180 ⁰F), to ensure the full heat treatment for the sealed containers.” ^[40]Ontario maple syrup production report. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 3 March 2016. Accessed March 2021 at
https://onmaplesyrup.ca/2016/03/03/ontario-maple-syrup-production-report/

One study showed that heating the jars first did allow the syrup put in it to stay hotter longer:

“Pre-heating of the 40 ml nip, 50 and 100 ml glass maple leaf bottles in the oven at 149° C (300° F) for 30 minutes or in a boiling water bath (4.5 cm depth of water) for 5 minutes increased the time syrup remained at temperatures greater than 70° C (158° F) to at least 3 minutes. This study showed that while bottling with syrup at 82° C to 85° C (180° F to 185° F) is a best management recommendation, modifications of bottling practices may need to be made to ensure smaller containers maintain a high enough temperature for a long enough period of time to kill fungal contamination.” ^[41]Maine Agricultural Center Integrated Research/Extension Grants 2012–2013. Page 3. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/resource/maine-agricultural-center-integrated-researchextension-grants-2012-2013/ .

It’s not just that the syrup and jars have to be within a certain temperature range at contact, that temperature range must be held for a bit in order to kill mould spores, because kill rate equals temperature plus time:

“To ensure mould-free syrup, each sealed container should be held at the hot temperature of 85° C  (185° F) for at least 5 minutes.” ^[42]Leuty, Todd. Preventing mould in packed maple syrup. Ontario Ministry of Agriculture, Food and Rural Affairs. 27 March 2014. Accessed March 2021 at http://www.omafra.gov.on.ca/english/crops/hort/news/hortmatt/2014/04hrt14a2.htm

No real lab research studies seem to have been done on heat / time / container pathogen kill rates with regards to maple syrup, as have long been done with home canning, so at this point it seems to be largely just best-guess:

“What are lacking are studies on how rapidly maple syrup cools in different containers. This information along with the temperature tolerances of maple syrup associated fungi will determine what bottling temperatures are necessary for maple syrup to be heated to prevent fungal contamination. The rate at which syrup cools from 85°C (185° F) to temperatures that fungi may survive will depend upon the container size, composition and the temperature in the surrounding environment.” ^[43]Maine Agricultural Center Integrated Research/Extension Grants 2012–2013. Page 1. Accessed March 2021 at https://umaine.edu/mainefoodandagcenter/resource/maine-agricultural-center-integrated-researchextension-grants-2012-2013/

An additional, though important, advantage of heating the glass jars first is to avoid giving cold glass jars thermal shock by pouring scalding hot syrup into them:

“For most hobby producers, the next step is simply to pour the syrup into containers. Containers must be clean. Mason or Ball jars work well, but pre-heat them with hot water to reduce the likelihood they crack from hot syrup.” ^[44]Smallidge, Peter. Cornell University Cooperative Extension. Getting Started with Small-Scale Maple Syrup Production. February 2018.

Quick cooling

After the jars and syrup have sat in the suggested spore kill range of 82° C to 85° C (180° F to 185° F) or higher, the syrup then needs to be cooled very quickly or syrup quality will be impacted:

“After a few minutes, the container should be cooled rapidly to minimize syrup darkening.” ^[45]Ontario Maple Syrup Production Report, April 19, 2018. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 19 April 2018. Accessed March 2021 at https://onmaplesyrup.ca/2018/04/19/ontario-maple-syrup-production-report-april-19-2018/

Keeping the syrup hot for too long after packing into jars results in a condition known as “stack burn”:

“Stack burn occurs where hot-packed maple syrup is kept at the hot temperature for an extended period beyond the normal sterilizing treatment. The syrup can continue to cook darkening the colour grade and affecting the flavour of syrup.  After the hot-pack period of 5 to 10 minutes, place the containers on a table spread apart in a well-ventilated area where they will cool down quickly.” ^[46]Maple syrup production report for March 24 to 30, 2014. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 24 March 2014. Accessed March 2021 at https://onmaplesyrup.ca/2014/03/24/maple-syrup-production-report-for-march-24-to-30-2014/

Here use of a fan is suggested:

“Cool the containers quickly. Upright the containers and spread them apart on a table to cool down fairly quickly to prevent continued cooking of the syrup.  A cool shaded area or a fan can help speed cooling.” ^[47]Ontario maple syrup production report. Ontario Ministry of Agriculture, Food and Rural Affairs. Ontario Maple Syrup site. 3 March 2016. Accessed March 2021 at https://onmaplesyrup.ca/2016/03/03/ontario-maple-syrup-production-report/

Note the caution against putting the hot glass jars into cold water. The need for the caution suggests that some have tried it, probably with unhappy results:

“Try to maintain the sealed container at the hot pack temperature for a period of 3 to 5 minutes before allowing it to cool. Then cool the hot containers quickly by placing them on a table, spread apart in a cool area. Some producers use an electric fan to cool the containers faster.  Never immerse hot glass containers into cold water, which could cause the hot glass to shatter.” ^[48]Maple Syrup Production report on March 13, 2015. 23 March 2015. Accessed March 2021 at https://onmaplesyrup.ca/2015/03/13/maple-syrup-production-report-on-march-13-2015/

Further reading

North American Maple Syrup Council. When is it syrup? (2017)